This paper describes the investigation of a passive ventilation solution to manage the hydrogen concentration within a large ullage space (0.9-3 m deep) above a liquid (free surface area of similar to 40 m(2)) containing a hydrogen source. The aim of the ventilation is to maintain the hydrogen concentration within the ullage space below 25% of the Lower Explosive Limit (LEL). The programme of tests involved examination of the ventilation performance in terms of sensitivity to chimney position, hydrogen release rate, hydrogen release point, ullage height and chimney diameter. The tests carried out lasted many hours, and the hydrogen concentration was monitored at a number of points within the ullage space. Pairs of ventilation chimneys with associated instrumentation systems were used to control and monitor the hydrogen concentration within the ullage space. This paper describes the approach to the testing, the results obtained and their analysis. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.